1. Field of the Invention
The present invention broadly relates to an internal combustion engine having a thermal air flowmeter. More particularly, the present invention relates to a device for detecting the transient state at which the temperature of intake air drastically changes, and also to an air-fuel mixture control device using such a detection device.
2. Description of the Related Art
A system has been known which computes fuel injection rate based on the intake air flow rate measured by a thermal air flowmeter and which executes fuel injection in accordance with the computed injection rate. This system suffers from a problem in that a disturbance is caused on the output of the thermal air flowmeter due to a drastic change in the intake air temperature, particularly when the intake air flow rate is small as in the case of idling, resulting in a variation in the air-fuel ratio and rough idling.
In order to overcome this problem, a system has been proposed which corrects the air-fuel ratio in accordance with the air temperature during idling of engine, as disclosed in Japanese Patent Laid-Open No. 57-49039, as well as a system employing a special temperature sensor responsive to drastic change in the intake air temperature and performing quick compensation for the change in the air temperature, as disclosed in Japanese Patent Examined Publication No. 62-12383.
These proposed systems, however, do not satisfactorily suppress the variation in the air-fuel ratio or rough idling, because the output of the thermal air flowmeter, which is the object of the temperature compensation, is unstable in the transient period in which the intake air temperature changes rapidly.
In addition, the temperature sensor employed in the system disclosed in Japanese Patent Examined Publication No. 62-12383 cannot determine whether disturbance a on the output of the thermal air flowmeter has taken place and is lasting due to a drastic change in the air temperature, although it can quickly detect such a drastic change in the air temperature occurring at a certain position.
Disturbance of the output from the thermal air flowmeter is a phenomenon which takes place, for example, under the conditions which will be described later. In some cases, the disturbance which is actually taking place may be attributed to a reason other than a presently detected slight change in the air temperature. In some other cases, disturbance of the output of the thermal air flowmeter is not caused even when the air temperature is changing rapidly. It is also considerable that the disturbance continues even after the air temperature at the measuring point has been settled. Thus, a mere detection of a drastic air temperature variation cannot provide any basis for exact determination of occurrence and state of disturbance in the output from the thermal air flowmeter.